A buoyancy control device (BCD) is a piece of diving equipment which is worn by divers to control buoyancy. A diver wears a BCD to adjust the effective weight of the diver in the water. A BCD is also called a buoyancy compensator (BC).
A BCD includes an inflatable bladder (or other similar device) that is inflatable orally or by a container of compressed gas. To rise up in the water, the bladder is filled with air, thus increasing the buoyancy of the diver. When the diver desires to sink in the water, gas is released from the bladder, thereby decreasing the buoyancy of the diver.
Divers use, for example, buoyancy compensating devices for adjusting buoyancy by the addition or release of air into the air chamber of the device. This air is usually introduced into the device by means of an inflator device, which is connected to the diver's breathing regulator and air supply via a low-pressure hose. Push button controls on the inflator device admit air to the buoyancy compensating device for positive buoyancy or vent air out for negative buoyancy. By convention, this inflator device is typically located on the left front shoulder of the device. The diver must adjust his or her buoyancy regularly because the air in the buoyancy compensating device compresses with depth, losing buoyancy as the diver descends, and conversely expands upon ascent, increasing the buoyancy.
In addition to the inflator device, there are vent valves to relieve excess pressure automatically or manually by means of a spring-loaded seal set to a specific over-pressure amount. These vent valves can be manually operated by means of a pull cord. When pulled, the pull cord pulls the vent valve away from its seat, venting air from the device. Many devices have a manual valve located behind the top right shoulder of the device, opposite the side where the inflator device is located. The pull cord attached to the vent valve travels along the edge of a right shoulder in a tube or fabric sleeve. Where the pull cord exits the tube or fabric sleeve, the pull cord terminates with a knob, ball, or some such feature that the diver can grasp and pull. Using the vent valve is usually easier and faster to manipulate than the push button inflator device.